Hi Shawn,If you have 3 axis, you can use only 3 sensors for the homeposition. In your software you can define the hardware-limits, both positive and negative. Your home position is not the same as the hardware machine limit.But if one of your axis is build with two separtely driven motors (Tandem-drive for your X or Y-axis) then you need 4 sensors.For the price of 2-3$ you can always buy one more: Murphy's law says: if you need 3 and buy 3 than one is probably malfunctioning, but if you buy 4, they will be all ok..

Proximity Sensors Compared: Inductive, Capacitive, Photoelectric, and Ultrasonic. Proximity sensors detect the presence or absence of objects using electromagnetic fields, light, and sound. There are many types, each suited to specific applications and environments.